Sprint said that during the first quarter, Amazon’s e-reader, the Kindle, drove a majority of the wireless carrier’s 394,000 new wholesale subscribers. But the big unknown is how much is Sprint getting for each one of those users?

Roger Entner, SVP, Nielsen’s Head of Research and Insights for Telecom, has released his best guess in a blog post, which pegs the average revenue per user at a mere $2 a month, putting into question how much Kindle-like devices could become a healthy new subscription model for Sprint (NYSE: S). In comparison, a direct postpaid Sprint subscriber has an ARPU of $56 a month. Entner cautions: “Because Kindle is included in Sprint’s overall subscriber numbers, the carrier will see pressure on its average revenue per user (ARPU) numbers going forward if ad hoc subscriptions become a significant business.”